%ICONURL{"toggleopen"}% gets you the image path: http://adjani.astro.umk.pl/foswiki/pub/System/DocumentGraphics/toggleopen.png
and that will get rendered as:

Customisation

To create a full new set of Icon's, you would need to duplicate this topic,
and then create a set of icon files matching these names as attachments to that copy.
This lengthy process is required to support direct URL usages of the iconsset
(its common for users to find out a url to an image, and then paste it into the editor - this 'just' works).

If you only want to customise a small set of icons, or add some of your own,
the best way to do this is to create an icon skin template containing your customisations,
and then to add that into your skin path.

eg:

Create a Skin Template for your icons

Either foswiki/templates/icons.mybook.tmpl on the filesystem, or create a topic called MybookSkinIconsTemplate in the System web
(or if the icon modifications are local to a web, the topic can be created in just that web).